1. Field of the Invention
The present invention relates generally to semiconductor device manufacturing technology, and particularly to a method for forming a metallization contact in a semiconductor device.
2. Description of the Related Art
Thin film aluminum and aluminum alloys are fundamental materials having application in the semiconductor integrated circuits industry. Aluminum is a good conductor, and adheres well to silicon and silicon dioxide. A significant problem in using aluminum for interconnects is junction spiking, which occurs at the interface of pure aluminum and silicon. This problem worsens when the interface is heated during commonly performed heat treatments, such as annealing, due to the change of the solubility of silicon aluminum with temperature. Junction spiking results in current leakage when the aluminum is a contact to a p-n junction. If aluminum penetrates beyond the p-n junction depth below the contact, the junction will be electrically shorted.
One technique to address to this challenge is to use a barrier metal structure, i.e., titanium/titanium nitride (Ti/TiN) double layer structure, as underlying layers for aluminum or aluminum alloy metallization contacts on silicon-based devices, in order to prevent the interdiffusion of aluminum and silicon, and to obtain a good ohmic contact at the interface thereof.
FIG. 1 shows a conventional method for forming a metallization contact. An insulating layer is formed on a semiconductor substrate 1 and etched to form a contact hole (not numbered) exposing an active region 2 of substrate 1. Reference numeral 2′ denotes a remaining portion of the insulating layer. A titanium (Ti) layer 3 is then deposited to cover insulating layer 2′ and active region 2 and a titanium nitride (TiN) layer 5 is deposited on Ti layer 3 in an nitrogen (N) atmosphere. An aluminum layer 6 is deposited on TiN layer 5 to form a metal wiring. Because titanium reacts with silicon in active region 2 to form titanium silicide (TiSix, generally TiSi2), a diffusion barrier having a structure of TiSi2/Ti/TiN is formed.
In the conventional aluminum metallization with the aforementioned diffusion barrier structure, the Ti and TiN layers are deposited by physical vapor deposition (PVD) sputtering method. However, PVD sputtering generally has inferior step coverage ability, and cannot provide adequate film thickness along the sidewalls of the contact hole. In particular, when an aspect ratio (i.e., the ratio of height-to-diameter) of the contact hole rises to be about 1, as integration of the device is increased, the step coverage of Ti/TiN decreases to be less than about 40%. Owing to the inferior step coverage, the electric resistivity of the metal wire increases during the operation of the device, resulting in decrease of the operational speed of the device, and over time, a short circuit condition between the active region of the substrate and the metal wire in the long run. In addition, the inferior step coverage of PVD may disturb the subsequent deposition of aluminum or aluminum alloys in the contact hole. Specifically, Ti/TiN deposited in the contact hole by PVD may have a negative slope, i.e., Ti/TiN deposited near a top corner of the contact hole is thicker than Ti/TiN deposited near a bottom corner of the contact hole, as shown in FIG. 1. As a result, subsequently deposited aluminum cannot provide sufficient coverage in the contact.
As an alternative to aluminum contacts, tungsten (W) has been used to form contacts, often referred to as “tungsten plugs.” Referring to FIG. 2, a contact hole (not numbered) is formed by etching a predetermined portion of an insulating layer 11 formed on substrate 10. A tungsten plug is formed in the contact hole. A TiN layer 12 is used as the barrier for the tungsten plug. In contrast with the above explained aluminum metallization, TiN barrier 12 is formed by a chemical vapor deposition (CVD) using TDMAT (Tetrakis Dimethylamino Titanium) as a source. Thus, the tungsten plug formation process does not incur junction spiking. Although the CVD TiN process has superior step coverage ability, however, certain precursor chemicals are required to initiate the formation of the TiN. Such precursor chemicals introduce a large quantity of impurities, e.g., carbon, into TiN layer 12. Carbon may diffuse into a silicon substrate 10, thus increasing the electric resistivity of the contacts.
Most impurities can be removed by plasma treatment, but plasma ions cannot reach the lower part of the sidewalls of the contact hole. The impurities in the lower part of TiN layer 12 on the sidewalls may result in the degradation of step coverage of the subsequent tungsten deposition process. FIG. 2 shows a void in the tungsten plug formed in the contact hole due to the nonuniformity in TiN layer 12. As a result, contact resistance increases and device reliability decreases.